Direct Chirped Laser Dispersion Spectroscopy (CLaDS) makes data analysis simple and straightforward, e.g. it enables fitting the dispersion profile using spectral databases (detailed description of CLaDS can be found in G. Wysocki and D. Weidmann, “Molecular dispersion spectroscopy for chemical sensing using chirped midinfrared quantum cascade laser,” Opt. Express, vol. 18, pp. 26123-26140, 2010—incorporated herein in its entirety). Unfortunately, it has also important drawbacks. One is related to the frequency demodulation noise that contributes significantly to the total noise in CLaDS. Due to quadratic dependence of the frequency demodulation noise with the acquisition bandwidth a trade-off between noise level and data sampling needs to be made. Second drawback is the presence of a residual baseline in the measured spectrum. The conventional CLaDS is baseline-free as long as frequency-shifted beams that are responsible for CLaDS signal generation travel the same distance (ΔL=0). In practice, however, ΔL≠0, unless some additional opto-mechanical stabilization is used. For the typical chirp rates being between 1014 and 1015 Hz/s the path difference of only ΔL=1 mm will result in the baseline level in the range of 50 to 500 Hz, whereas typical signal amplitude for the trace-gas sensing varies from tens of Hz to several tens of kHz, depending on the target molecular transition, molecular concentration, optical path length within the sample etc. Moreover, when the triangular modulation is used the resulting frequency chirp is not linear, thus the correct subtraction of the baseline requires fitting using higher order polynomials. Both issues can be minimized or eliminated when CLaDS signal is extracted using a Chirp Modulation (CM-CLaDS) scheme.
The present disclosure relates to chirped laser dispersion spectroscopy (CLaDS) systems. By applying the process disclosed herein, the baseline in the dispersion measurement may be reduced and measurement is made more immune to opto-mechanical fluctuations. In addition, by applying the process disclosed herein, the noise level may be reduced and signal to noise ratio can be improved compared to the conventional method. The process disclosed herein may be used in spectroscopic applications in which continuous monitoring of the molecule concentration is needed. The method may be used with single point extractive sensing systems as well as with open path remote sensors based on CLaDS.